* docs(hub): drop the manual auth secret from self-hosting setup
Hub generates its authentication secret on first start and stores it in the
database it is already using, so self-hosters no longer run openssl or carry
PASEO_HUB_AUTH_SECRET between restarts. The variable stays as an override for
deployments that must supply the secret from a platform secret store; Hub uses
it without persisting it, so removing it resumes the stored value.
Also corrects a claim that predates this change: rotating the secret does not
invalidate execution credentials already issued. Hub verifies those against
persisted hashes rather than the secret, so only browser sessions end.
Documentation companion to https://github.com/getpaseo/hub/pull/50.
* docs(hub): lead with the local guided setup
* docs(hub): document XDG data directory
* docs: make Hub self-hosting the quickstart
